The present invention relates to color filters and liquid crystal display devices (or display elements, display units). More particularly, the invention relates to color filters and liquid crystal display devices which are used for portable data terminals or the like and further relates to color filters and liquid crystal display devices of types usable for both transmission and reflection.
Liquid crystal display devices are roughly divided into transmissive type and reflective type devices.
First, with regard to transmissive type devices, commonly used transmissive type color liquid crystal display devices employ absorption type color filters. The transmissive type color liquid crystal display device has a structure as shown in FIG. 1. In the figure, the reference numeral 11 denotes a light guide plate provided with a reflection plate (not shown in the figure) on its lower side or the like. The reference numeral 12 denotes a polarizer. The reference numeral 14 denotes a back (lower) side substrate. The reference numeral 15 denotes a front (upper; viewers operate and watch the display screen there) side substrate. The reference numeral 17 denotes a polarizer on the front side. The reference numeral 61 denotes an absorption type color filter for transmitting red light (absorbing light of wavelengths other than red). The reference numeral 62 denotes an absorption type color filter for transmitting green light. The reference numeral 63 denotes an absorption type color filter for transmitting blue light. The reference numeral 21 denotes a light source as a sidelight or a backlight.
Basic function, structure, material and the like of each of these portions are so-called well-known arts; therefore general descriptions thereof are not provided herein.
Actually, in addition to the above, TFT (Thin Film Transistor) for pixels and for driving pixels is formed on either of the upper and lower substrates, and transparent conductive film and alignment film are disposed on the sides on which those substrates face a liquid crystal layer. The TFT, the transparent conductive film and the alignment film also are so-called well-known arts and additionally are not directly related to the scope of the present invention; therefore illustrations and descriptions thereof are not provided herein.
Only contents, functions and the like which are directly related to the scope of the present invention are described below in principle.
The backlight disposed on the back side of a liquid crystal panel is a so-called white light source (which comprises light of any wavelength so as not to make light of particular colors observable).
Each pixel is provided with an absorption type color filter which transmits only predetermined light corresponding to a color of the pixel and absorbs light other than the predetermined light. Thus, each color of red, green and blue is displayed.
The use of the absorption type color filter, however, causes an extremely great loss of light due to the absorption of approximately two-thirds (or more in some cases) of the light. Accordingly, it has been proposed that an interference filter should be substituted for the absorption type color filter (for example, Japanese Unexamined Patent Publications No. 11-38397, No. 10-197859 and Japanese Patent Application No. 9-273042). An interference filter transmits light of particular colors and reflects light other than the light of particular colors, in other words, reflects light of unnecessary colors for the pixel on the side of the light source. This reflected light thereby is reflected again in a display surface by a reflection plate or the like on the lower side of the light guide plate 11 to be reused for display, resulting in the improvement of utilization efficiency of light as well as the decrease of power consumption and the improvement of luminance.
An example such that an interference filter is applied to a reflective type liquid crystal element is disclosed in Japanese Unexamined Patent Publication No. 11-38397, a model such that an interference filter is formed under a color filter is disclosed in Japanese Unexamined Patent Publication No. 10-197859, and an example of a backlight for color separation using an interference filter is disclosed in Japanese Patent Application No. 9-273042.
An interference filter is generally formed with a multilayer film, while a type such that an interference filter is formed by curing cholesteric liquid crystal has been proposed in recent years (for further details, refer to xe2x80x98the development of color filters by using cholesteric liquid crystal presented by Tokuhisa MORIYA in DAI NIPPON PRINTING CO., LTD. at the 6th research report meeting of HLCxc2x0).
Next, with regard to reflective type devices, commonly used reflective type color liquid crystal display devices also employ absorption type reflection plates. The reflective type color liquid crystal display device has a structure shown in FIG. 2 such that a reflection plate 71 which reflects only predetermined light and absorbs light other than the predetermined light is formed on a substrate on the back side of a liquid crystal panel. Thus, only pixels for brightly displaying predetermined light reflect the predetermined light; the reflection plate reflects the predetermined light on the display side (the side of viewers);.and only the pixels for brightly displaying predetermined light absorb the predetermined light, while a circular light plate and a liquid crystal layer shut off (absorb) the predetermined light in the case of pixels for not brightly displaying predetermined light. In FIG. 2, corresponding numbers are given to members and portions having the same function, structure and the like as in FIG. 1. Similarly, these numbers apply to liquid crystal display devices according to embodiments of the present invention described after. The reference numeral 71 in FIG. 2 is a reflection plate as well as an electrode.
It is noted for confirmation that transmitted light in this type passes back and forth through a liquid crystal layer, namely, passes through twice for display (this is related to embodiments described after).
In addition to the above, partially transmissive type liquid crystal display devices are mentioned as an intermediate type between the transmissive type and the reflective type. This type is such that a half mirror is disposed on the upper or lower side of the back side substrate 14 of the liquid crystal display device shown in FIG. 1 to transmit a certain quantity of light and reflect a certain quantity of light. A liquid crystal display element of this type is used as the transmissive type when an AC power supply can be used and is used as the power-saving reflective type when battery-operated, being widely used particularly for portable data terminals or the like.
Furthermore, a type of device usable as both a transmissive type and a reflective type device was announced in recent years (in a news release by SHARP CORPORATION on Apr. 1, ""99). This type of device is an improved conventional partially transmissive type device and is characterized in that a reflection plate is partially formed at every pixel.
Brightness, however, is not sufficiently secured in liquid crystal display elements described above. Since polarized light has, from the start, been utilized in liquid crystal display devices, utilization efficiency of light is reduced to approximately a half in principle even for mere black-and-white display.
In particular, brightness is further reduced in a type of device usable as both a transmissive type and a reflective type device. For example, if, for the sake of simplicity of explanation, transmittance of a half mirror is assumed to be 50%, only 50% of brightness of a backlight is utilized in the case of transmissive type display, while only 50% of reflected light is utilized in the case of reflective type display. Resultingly, utilization efficiency of light is reduced to half in both types of display. This reduction is caused also when a reflection plate is formed at pixels (actually, in use as a transmissive type device employing a backlight, the transmittance will frequently be less than 50% as compared with use as a reflective type device because of having extra possible luminance).
Moreover, an absorption type color filter is used for a color filter so that two-thirds (or more in some cases) of light is not utilized; therefore utilization efficiency of light is reduced to approximately one-sixth in principle for color display. The use of the above-mentioned half mirror can reduce utilization efficiency of light to approximately one-twelfth in the worst case.
In a model such that an interference filter is used as a measure against the reduction, the display screen is observed in such a manner that colors thereon change when viewing from an oblique direction. That is, an interference filter is formed with a multilayer film or the like having periodicity and selectively reflects light of wavelengths corresponding to the thickness thereof. Consequently, the effective thickness of the interference filter is increased when the display screen is viewed from an the oblique direction, leading to a change in wavelengths which are transmitted.
Less power consumption and brighter display, therefore, have been desired in transmissive type devices and reflective type devices as well as particularly a type usable as both of the types and additionally a liquid crystal display device for color display. It has been desired that colors not change with the direction from which the display screen is viewed and that color saturation be improved.
The present invention is intended for solving the above-mentioned problems and employs a nonabsorption type color filter as a color filter.
According to the invention, transmissive type display and reflective type display are performed.
According to the invention, a back side light source such as a backlight and a sidelight can be switched off independently of a pixel unit for use of the device as a reflective type device.
According to the invention, an interference filter is used as the nonabsorption type color filter.
According to the invention, although modulation characteristics of a liquid crystal layer differ between a transmissive type mode and a reflective type mode in accordance with the thickness of the liquid crystal layer through which light passes through (the distance of transmission), either mode offers a bright display and excellent contrast.
According to the invention, a color filter is an interference filter formed with a multilayer film, and additionally the normal direction of the multilayer film differs from the normal direction of a substrate.
More specific descriptions are detailed below.
An aspect of the invention is characterized in that a liquid crystal display device for color display (including a projection display) having a back side light source, such as a backlight or a powerful light source for projecting, employs as a color filter corresponding to a color of each pixel making up part of a delta, a mosaic, a stripe or the like on a display surface (in the case of projection display, strictly speaking a surface as a film used for displaying) not a conventional color filter which absorbs light other than light of predetermined colors by dispersing colorant into transparent resin, but a nonabsorption type color filter which transmits light of predetermined colors and reflects light other than the light of predetermined colors (note that the nonabsorption type color filter is not limited to the display of the three primary colors).
Another aspect of the invention is characterized in that a liquid crystal display device is a combination liquid crystal display device (as described above, including elements and display units of portable apparatuses) capable of switching between a transmissive type display mode and a reflective type display mode by switching a switch.
A further aspect of the invention is characterized in that the quantity of light of a backlight can be shutoff (to zero) for the use as a reflective type device.
A still further aspect of the invention is characterized in that a nonabsorption type color filter is an interference color filter, such as one formed with a multilayer film, that transmits only light of predetermined colors such as red and reflects light of complementary colors thereto.
A still further aspect of the invention is characterized in that color display of red, green and blue is performed in transmissive type display and color display of cyan, magenta and yellow is performed in reflective type display. For this purpose, the device comprises a predetermined switching circuit and the like for such color displays.
A still further aspect of the invention is characterized in that a display can be inverted to a negative by shifting a TFT between ON and OFF in order to perform an optimum display for each of transmissive type display and reflective type display.
A still further aspect of the invention is characterized in that a liquid crystal display device which can switch between a transmissive type display mode and a reflective type display mode comprises reflection means for reflecting incident light entering from the display side or light of a predetermined color by each pixel for white display and color display when the device is used as a reflective type device, and transmission means for transmitting light of white color or light of a predetermined color by each pixel when the device is used as a transmissive type device.
A still further aspect of the invention is characterized in that a voltage signal for modulation in transmissive type display is set to be large in accordance with a type of liquid crystal to be used therefor in order to compensate for the fact that the amount of modulation, change in the polarization angle of light for display by a liquid crystal layer, is less as compared with reflective type display.
A still further aspect of the invention is characterized in that a portion of a cell is thickened in order to thicken the liquid crystal layer of the portion through which light for display passes when the device is used as a transmissive type device.
A still further aspect of the invention is characterized in that a voltage for modulation is raised in order to increase the amount of modulation in use as a transmissive type device and in that a liquid crystal is used which greatly modulates in response to a high voltage.
A still further aspect of the invention is characterized in that reflected light by a reflection means and transmitted light by a transmission means are in a complementary color relationship and that modulation by a liquid crystal layer can be switched by switching display mode.
A still further aspect of the invention is characterized in that a reflection means and a transmission means are nonabsorption types and particularly interference filters.
A still further aspect of the invention is characterized in that an interference filter is formed with a multilayer film and that the multilayer film is formed with inclinations on a surface of the multilayer film at each pixel in the horizontal, vertical, or the like directions as viewed by a viewer in order not to cause disorder to color display depending on the viewing angle.
A still further aspect of the invention is characterized in that a cholesteric liquid crystal is used for the multilayer film.
A still further aspect of the invention is characterized in that an interference filter using the cholesteric liquid crystal is a complex comprising a cholesteric liquid crystal and a polymer.
A still further aspect of the invention is characterized in that a liquid crystal display device which is not a type usable as both a transmissive type device and a reflective type device employs a nonabsorption type color filter for full color display, dichroic display, monochromatic display and the like and that the same techniques as in each of the above-mentioned groups of inventions are employed for the nonabsorption type color filter.